A vinyl chloride-based polymer, which is a polymer containing 50 wt % or more of a repeat unit derived from a vinyl chloride monomer (VCM), is widely applied because a cost is low, hardness is easily adjusted, and it is applicable to most processing devices. Moreover, a vinyl chloride-based polymer may be used to prepare a molded product excellent in physical and chemical properties, for example, mechanical strength, weather resistance, chemical resistance, and the like, and thus has been widely used in various fields.
Meanwhile, in the preparation of a vinyl chloride-based polymer with a low degree of polymerization, polymerization is generally performed at higher temperature and higher pressure than in the preparation of a vinyl chloride-based polymer with a high degree of polymerization. However, it is difficult to safely prepare a vinyl chloride-based polymer with a low degree of polymerization because of a limitation in a design pressure of a polymerization reactor and the like.
Therefore, a degree of polymerization may be adjusted using a mercaptan-based chain transfer agent, but this method has a drawback in that it is difficult to achieve a desired degree of polymerization or a polymer having a wide molecular weight distribution is obtained because, when a vinyl chloride-based polymer is prepared through bulk polymerization, polymerization is performed without water according to characteristics of bulk polymerization, and thus the dispersibility of an added chain transfer agent is degraded.
Accordingly, in the preparation of a vinyl chloride-based polymer with a low degree of polymerization, technology for preparing a polymer having a low polydispersity index and a desired degree of polymerization due to a narrow molecular weight distribution even while a safe operating condition is maintained upon the preparation is required.